Absolute configuration of D(2)-symmetric fullerene C(84)

The D(2)-symmetric isomer of fullerene C(84) predominantly found in soots is inherently chiral. We determine its absolute configuration by comparison of the experimental electronic circular dichroism (CD) spectrum with time-dependent density functional calculations.     

Face-driven corner-linked octahedral nanocages: M6L8 cages formed by C3-symmetric triangular facial ligands linked via C4-symmetric square tetratopic Pd(II) ions at truncated octahedron corners

The face-driven corner-linked truncated octahedral nanocages, [Pd6L8]12+ (1, L1 = N,N',N' '-tris(3-pyridinyl)-1,3,5-benzenetricarboxamide; 2, L2 = N,N',N' '-tris(4-pyridinylmethyl)-1,3,5-benzenetricarboxamide), were prepared with eight C3-symmetric tridentate ligands and six square planar tetratopic palladium(II) ions. The combination of the nitrogen donor atom at a approximately 120 degrees kink position of the carboxamido pyridinyl group and the tilted pyridyl versus the facial plane of the ligands can provide the needed curvature for the formation of octahedral cages. The nitrogen atoms can coordinate to the square planar palladium(II) ions to form kinks with approximately 120 degrees angles at the C4-symmetric square planar corners of the truncated octahedron. Depending on the conformation of the ligand, L1, two different truncated octahedral cages of around 2.4 nm in diameters were formed. The major form of 1 with syn-conformational ligands has a cavity volume of approximately 1600 A3. The cage has 12 ports (3.4 x 3.5 A2) at all edges of the octahedron. The minor form of cage 1 with anti-conformational ligands has a slightly increased cavity volume ( approximately 1900 A3) and port size (3.3 x 8.0 A2). The insertion of a methylene group in L2 has not only increased the cavity volume of 2 to approximately 2200 A3 but also enlarged the port size to 4.1 x 8.0 A2. However, an atomic force microscopy (AFM) study of cage 2 showed that the cages had a height of 1.8 +/- 0.1 nm. This value is about 30% smaller than the calculated size of 2.6 nm from the crystal structure. This tip-induced decrease in height in cage 2 suggests the nonrigidity of cage 2.     

Directed assembly of chiral oxidovanadium(V) methoxides into C4-symmetric metal(I) vanadate-centered quadruplexes: synergistic K+- and Ag+-specific transport

A directed assembly process to form C4-symmetric, vanadate-centered quadruplexes, for the first time, from a given chiral oxidovanadium(V) complex allows for highly efficient K+- and Ag+-specific transport from aqueous phase containing three other alkali metal cations into organic solvents, reminiscent of the K+ specific transport exerted by four homochiral glycine residues of the opening site in KcsA membrane protein.     

Synthesis and columnar mesophase of fluorescent liquid crystals bearing a C2-symmetric chiral core

A chiral and fluorescent columnar mesogen prepared from chiral binaphthols is reported. This liquid crystal comprises a C2-symmetric chiral core with two staggered aromatic planes. Its hexagonal columnar (Col(h)) mesophase was characterized by appropriate physical methods.     

Synthesis of C3-symmetric tris(beta-hydroxy amide) ligands and their Ti(IV) complex-catalyzed enantioselective alkynylation of aldehydes

[reaction: see text]. A series of new chiral C3-symmetric tris(beta-hydroxy amide) ligands have been synthesized via the reaction of 1,3,5-benzenetricarboxylic chloride and optically pure amino alcohols (up to 96% yield). The asymmetric catalytic alkynylation of aldehydes with these new C3-symmetric chiral tris(beta-hydroxy amide) ligands and Ti (O(i)'Pr)4 was investigated. Ligand 4c synthesized from (1R,2S)-(-)-2-amino-1,2-diphenylethanol is effective for the enantioselective alkynylation of various aldehydes, and high enantioselectivity was obtained with aromatic aldehydes and alpha,beta-unsaturated aldehyde (up to 92% ee).     

Design and synthesis of C3-symmetric Lewis(x) antigen

[structure: see text] C(3)-Symmetric glycoconjugates carrying three equivalent Lewis(X) antigens or beta-lactosides were synthesized from p-nitrophenyl glycosides and trimesic acid via regio- and stereocontrolled glycosylation reactions. An (1)H NMR study has shown that the C(3)-symmetric glycoconjugates soluble in water provide useful probes to investigate the Ca(2+)-dependent Lewis(X)-Lewis(X) association.     

Enantiopure C1-symmetric bis(imino)pyridine cobalt complexes for asymmetric alkene hydrogenation

Enantiopure C(1)-symmetric bis(imino)pyridine cobalt chloride, methyl, hydride, and cyclometalated complexes have been synthesized and characterized. These complexes are active as catalysts for the enantioselective hydrogenation of geminal-disubstituted olefins.     

Structurally well-defined, recoverable C3-symmetric tris(beta-hydroxy phosphoramide)-catalyzed enantioselective borane reduction of ketones

[reaction: see text] A series of new chiral C(3)-symmetric tris(beta-hydroxy phosphoramide) ligands have been synthesized via the reaction of trisphosphoramide ester and Grignard reagents. The catalytic asymmetric borane reduction of ketones with these new C(3)-symmetric chiral tris(beta-hydroxy phosphoramide)s was investigated. Structurally well-defined, recoverable ligand 1d is an efficient catalyst for the enantioselective borane reduction of both electron-deficient and electron-rich ketones, and high enantioselectivities were achieved (up to 98% ee).     

Design, synthesis and DNA-cleaving efficiency of photoswitchable dimeric azobenzene-based C2-symmetric enediynes

Designed azobenzene-based enediyne-amino acid C(2)-symmetric hybrids have been synthesized and the role of amino acid linker in stabilizing the Z form has been demonstrated; DNA-binding and cleavage studies have established higher reactivity of the Z-isomers.     

Stable supramolecular helical structure of C6-symmetric hydrogen-bonded hexakis(phenylethynyl)benzene derivatives with amino acid pendant groups and their unique fluorescence properties

A highly stable supramolecular helical structure was formed by the self-assembly of novel C6-symmetric hydrogen-bonded discotic molecules, hexakis(phenylethynyl)benzene derivatives with chiral alanine parts, and exhibited orange excimer emission with a large Stokes shift.     

A new approach to enantiopure C3-symmetric molecules

Chiral base chemistry has been used to create three chiral centres in one pot on a C3-symmetric substrate. The potential of this new approach to C3-symmetric molecules is exemplified by the creation of an enantiopure C3v-symmetric triol, triphosphane and tripyridine. A ruthenium complex of the last compound has been studied by X-ray crystallography.     

Kinetic control in the palladium-catalyzed synthesis of C(2)-symmetric hexabenzotriphenylene. A conformational study

The hitherto unisolated, thermodynamically unstable C(2)-symmetric conformer of hexabenzotriphenylene (1) has been efficiently synthesized by palladium-catalyzed cyclotrimerization of 9, 10-didehydrophenanthrene (4). The barriers to conformational interconversion of 1 are examined experimentally and by computational studies.     

Asymmetric Henry reaction catalyzed by C2-symmetric tridentate bis(oxazoline) and bis(thiazoline) complexes: metal-controlled reversal of enantioselectivity

[reaction: see text] C(2)-symmetric tridentate bis(oxazoline) and bis(thiazoline) ligands with a diphenylamine backbone have been investigated in the catalytic asymmetric Henry reaction of alpha-keto esters with different Lewis acids. Their Cu(OTf)(2) complexes furnished S enantiomers, while Et(2)Zn complexes afforded R enantiomers, both of them with higher enantioselectivities (up to 85% ee). Reversal of enantioselectivity in asymmetric Henry reactions was achieved with the same chiral ligand by changing the Lewis acid center from Cu(II) to Zn(II). The results show that the NH group in C(2)-symmetric tridentate chiral ligands plays a very important role in controlling both the yields and enantiofacial selectivity of the Henry products.     

Asymmetric Michael addition of nitroalkanes to nitroalkenes catalyzed by C2-symmetric tridentate bis(oxazoline) and bis(thiazoline) zinc complexes

The first asymmetric synthesis of 1,3-dinitro compounds through Michael addition of nitroalkanes to nitroalkenes catalyzed by C2-symmetric chiral tridentate bis(oxazoline) and bis(thiazoline) zinc complexes was achieved with high enantioselectivities (up to 95% ee).     

Bowl-shaped C(3)-symmetric receptor with concave phosphine oxide with a remarkable selectivity for asparagine derivatives

A bowl-shaped C(3)-symmetric receptor (1a) that has a phosphine oxide functionality in the interior of a "molecular bowl" shows remarkable selectivity for Asn derivatives. [structure: see text]     

Synthesis of a D3-symmetric "trefoil" knotted cyclophane

A D(3)-symmetric knotted cyclophane, with a subgraph of trefoil topology, was synthesized by cyclization of a D(3)-symmetric scaffold. Control of configuration at the three metal-based stereocenters arises from a bascule/pivot mechanism.     

Efficient synthesis of enantiomerically pure trans-2,5-bis(arylethynyl)pyrrolidines. A new entry into C2-symmetric chiral secondary amines

A new class of C₂-symmetric pyrrolidine derivatives bearing arylethynyl groups at the 2,5-positions has been synthesized in enantiomerically pure form from 1,7-octadiyne-3,6-diol in five steps. Some notable features of the synthesis are: (i) the formation of a separable diastereomeric mixture of pyrrolidine carbamates using a newly prepared chiral chloroformate; and (ii) the development of a new method for the deprotection of the carbamate via a novel SmI₂-promoted electron transfer process.     

Synthesis and properties of novel C3-symmetric coplanar chromophores

New synthetic pathways for novel C(3)-symmetric molecules featured with a thiophene-fused six-five-five carbon-bridged coplanar core structure have been established. The incorporation of thiophene as the constituent of a C(3)-symmetric core provides effective modulation of the physical properties and imparts extra flexibility for π-conjugated functionalization stemming from either the embedded thiophene or pendant aryl substitutions.     

Synthesis and reactivity of a C3-symmetric trinuclear zinc(II) hydroxide catalyst efficient at phosphate diester transesterification

Inspired by trinuclear Zn(ii) sites in enzymatic systems, a ligand system containing three preorganized (2-pyridyl)methyl piperazine moieties anchored onto a rigid C(3)-symmetric triphenoxymethane platform has been developed for preorganizing three zinc ions into an environment conducive to intramolecular interaction. Zinc(ii) binding by this ligand has been analyzed by means of potentiometric measurements in 50% (v/v) CH(3)CN-H(2)O solutions. Subsequently a C(3)-symmetric trinuclear Zn(ii) hydroxide complex of the C(3)-symmetric ligand was synthesized and fully characterized using NMR spectroscopy and X-ray crystallography. This complex induces a 16 900-fold rate enhancement in the catalytic cyclization of the RNA model substrate, 2-hydroxypropyl-p-nitrophenyl phosphate (HPNP, pH 6.7, 25 degrees C) over the uncatalyzed reaction with multiple catalyst turnovers. The observed differences in the pH-rate profile can be attributed to the varying concentration of various trinuclear zinc species. The trinuclear Zn(ii) catalyst exhibits a higher hydrolytic activity compared to its mononuclear analogue. The reactivity and structural features of this trinuclear Zn(ii) complex will be discussed.     

Synthesis and structure of preorganized,C(3)-symmetric trilactam scaffolds with convergently oriented (S)-acetylthiomethyl appendages

[reaction: see text] Efficient modular synthesis of conformationally preorganized, C(3)-symmetric trilactams is reported. The allyl acetate cyclization substrate was synthesized in five steps from Garner's L-serine-derived aldehyde. After chiral ligand-mediated palladium cyclization, the resulting vinyl hydropyran was transformed into the orthogonally protected amino acids for iterative coupling. The final macrolactamization was accomplished using EDCI/HOBt or HATU/HOAt under high dilution conditions.